security/libssh: Mark BROKEN on 15+ and add backup for MASTER_SITES

Fails to build on 15+,
src/mlkem_crypto.c:31:10: fatal error: 'openssl/ml_kem.h' file not found

Add temporary backup for MASTER_SITES to avoid build failures on other
versions and of consumers as main upstream site current truncates
downloads

Thanks to eduardo@ for verifying build issues on -CURRENT

PR: 294268
Approved by: blanket, just fix it